1. Field of the Invention
The present invention relates to a wavelength-division-multiplexed (WDM) optical transmission system, and more particularly to a light source employed in the WDM optical system.
2. Description of the Related Art
A wavelength-division-multiplexed passive optical network (PON) allocates a wavelength for each of the subscribers, thus offering a high-speed broadband communication service. In particular, communication confidentiality is ensured, and a required additional communication service or an expansion in the communication capacity can be accommodated easily to the subscribers. In addition, an increase in the number of subscribers can be serviced easily by assigning a new inherent wavelength to each new subscriber. Despite these advantages, as a light source having a specific wavelength and an additional wavelength stabilizing circuit for stabilizing the light source are required at the central office (CO) and for each of the subscriber stages, the WDM PON is expensive to implement. Thus, there is a need to develop a cost effective WDM light source for the WDM PON. A distributed feedback laser array (DFB laser array), a multi-frequency laser (MFL), a spectrum-sliced light source, and a wavelength-locked Fabry-Perot laser with incoherent light, etc., have been suggested as WDM light sources. The spectrum-sliced light source, which is being actively researched, can offer a number of wavelength-division channels by spectrum-slicing a broadband optical signal with an optical filter or a waveguide grating router (WGR). As such, this type of light source does not require the wavelength selectiveness and the wavelength stabilization.
A light emitting diode (LED), a super-luminescent diode (SLD), a Fabry-Perot laser (FP laser), a fiber amplifier light source, and an ultra short pulse light source, etc., have been also suggested as spectrum-sliced light sources. The wavelength-locked Fabry-Perot laser with incoherent light is configured to spectrum-slices a broad bandwidth optical signal, which has been produced from an incoherent light source, such as a light emitting diode or a fiber amplifier light source, using an optical filter or waveguide grating router, then uses a wavelength-locked signal for transmission, which is outputted by inputting the sliced signal into a Fabry-Perot laser that is not equipped with an isolator. When a spectrum-sliced signal having an output greater than a predetermined value is inputted into the Fabry-Perot laser, the Fabry-Perot laser produces and outputs only a wavelength equal to that of the inputted spectrum-sliced signal.
Meanwhile, each of the distributed feedback (DFB) laser array and the multi-frequency laser (MFL) requires a complicated manufacturing process and utilizes an expensive device that requires a light source having an accurate wavelength selectiveness and wavelength stabilization for wavelength division multiplexing. Although the light emitting diode (LED) and the super-luminescent diode (SLD) have a very broad light bandwidth are inexpensive, they are only suitable for a light source for an upward signal, which has a low modulation rate compared to a downward signal, as their modulation bandwidths and outputs are low.
The Fabry-Perot laser is an inexpensive, high-power device. However, it has disadvantages in that it cannot offer many wavelength-division channels due to its narrow bandwidth and that in the case of modulating and transmitting a spectrum-sliced signal with high speed, a performance degradation caused by a mode partition noise is great.
The ultra short pulse light source is coherent and has a very broad spectrum band. However, it is difficult to function as light source as the stability of the oscillated spectrum is poor and the pulse width is no more than several ps.
As an alternative to the above-described light sources, a spectrum-sliced, fiber amplified light source has been introduced to spectrum-slices an amplified spontaneous emission light (ASE light) produced from the fiber amplifier. The spectrum-spliced light source is capable of offering many high-power wavelength-division channels. However, it must use an expensive, independent external modulator, such as a LiNbO3 modulator, so that each channel may transmit different data. In contrast, the wavelength-locked Fabry-Perot laser with incoherent light directly modulates the Fabry-Perot laser depending on the data signal, thus can more economically transmit the data. However, the Fabry-Perot laser requires input of a broad bandwidth, high-power incoherent light signal so that the Fabry-Perot laser may output a wavelength-locked signal that is suitable for a high-speed, long distance transmission. And, it is impossible to make a long distance transmission due to a dispersion effect of the optical fiber as the signal of the Fabry-Perot laser, which is self-seeded and outputs when an incoherent light having a bandwidth broader than a mode interval of the output signal of the Fabry-Perot laser is inputted for high speed transmission, becomes a signal with a plurality of wavelengths distributed depending on the mode interval,
Accordingly, there is a need for an improved WDM light source that can address the drawbacks described in the preceding paragraphs.